1. Field of the Invention
This invention relates to improvements in methods and circuits for driving brushless, polyphase dc motors, such as a spindle motor of a hard disk drive, or the like, and more particularly to improvements in such driving methods and circuits that at least reduce the acoustic and electrical noise in motors of this type using driving voltages having substantially sinusoidal or trapezoidal waveforms.
2. Related Background
In the operation of a polyphase dc motor, a dc current is commutatively applied to the coils of the motor. In the past, the current that is applied has a square waveform when it is switched from one coil set to the next. The square waveform produces many high frequency components, and, additionally, requires large amounts of power. As the power requirements for hard disk drives and the like are reduced, much emphasis has attached to efforts to reduce the driving currents required for the motor. Each square wave commutation results in many voltage and current spikes, which are undesirable.
The physical size of the drives also is being reduced, for example, for use in laptop computers, or the like. The reduced sized drives use less power, for example, due to the lessened need of drives of smaller size for such high commutation currents in operation. However, as the voltage of the hard disk drive is reduced, the drive becomes more sensitive to noise spikes.
Typically, the circuitry to spin the motor of the drive is contained on the same circuit board as the read channel electronics, which process the signals read from the mass data storage device. As a result, noise that occurs in the commutation/driver electronics may translate into noise that is undesirably transferred to the electronics in the read channel. As a result, it is desirable to reduce the noise and commutation spikes in the commutation/driver circuit as much as possible.
Efficient motor driver requires that the excitation current in the three motor phases be aligned with the back-emf generated on the three phases. One of the best schemes for achieving this alignment is the use of a phase-locked loop (PLL) that adjusts the phase and frequency of the commutation, so the back-emf of the un-driven winding passes through zero in the center of the appropriate commutation state. This scheme works well when the shape of the commutation waveforms includes an un-driven region, as in a conventional 6-state, +1, +1, 0, xe2x88x921, xe2x88x921, 0, sequence.
Since the +1, +1, 0, xe2x88x921, xe2x88x921, 0 sequence has sharp transitions between driving states, this sequence has many high frequency components. In addition to the undesirable effects discussed above, the high frequency components tend to excite mechanical resonance in the motor, which results in the creation of undesirable acoustic noise. Moreover, the step-function tristating of the undriven motor phase, together with the step-function driving waveform produce a degree of torque ripple in the motor. The torque ripple results in an unevenness or jerkiness in the motor rotation, which also excites resonance in the motor, also causing undesirable acoustic noise.
There has been recent emphasis on disk drive manufacturers to reduce the noise associated with disk drive motors. Consequently, what is needed is a disk drive and method for operating it in which the noise associated with the driver in operation is reduced or eliminated.
In light of the above, therefore, it is an object of the invention to provide an improved motor driver and method for operating it in which the noise associated with the driver in operation is reduced or eliminated.
It is still another object of the invention to provide a disk drive and method that employs multiple sinusoidal or trapezoidal drive signals, or the like, for application to the motor windings.
It is yet another object of the invention to provide a sinusoidally or trapezoidally shaped waveform for commutative switching and application to the coils of a polyphase dc motor.
It is still another object of the invention to provide a polyphase dc motor driver and method of operating it in which sinusoidally or trapezoidally shaped waveforms are commutatively applied to enable smoother and quieter operation with reduced torque ripple.
These and other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims.
Thus, in accordance with a preferred embodiment of the invention, a spindle current control system for use in a three-phase dc motor is presented. The control system provides a means to accurately control the spindle current in a user-defined waveform for minimizing acoustic/electrical noise and torque ripple. A method is also provided to generate a linear current ramp waveform is also presented. In addition, a circuit to generate a slew rate that is independent of variations in process and temperature is presented. A method to enable or disable each individual power amplifier associated with three phase windings is also presented.
In accordance with a broad aspect of the invention, a circuit is presented for providing drive currents to a polyphase dc motor. The circuit includes a circuit for generating a user-defined waveform first signal in synchronism with a reference frequency signal. A reference frequency signal generator generates pulses of a second signal at a reference frequency. A commutation circuit combines segments of the first and second signals, and a circuit is provided for applying the combined sinusoidal or trapezoidal waveforms to coils of the polyphase dc motor in a commutative sequence. Preferably, the user-defined waveform, when combined with the second signal, has approximately a sinusoidal or trapezoidal waveform.
According to another broad aspect of the invention, a method is presented for operating a polyphase dc motor. The method includes generating a commutation waveform, generating a user-defined waveform in synchronism with the commutation waveform, and combining the commutation waveform and the user-defined waveform to produce a combined waveform. The combined waveform is then applied to the windings of the motor. The combined waveform, preferably, has approximately a sinusoidal or trapezoidal waveform.
According to still another broad aspect of the invention, a method is presented for reducing acoustic noise in operating a polyphase dc motor. The method includes generating a commutation waveform, generating a user-defined waveform in synchronism with the commutation waveform, and combining the commutation waveform and the user waveform to produce a combined waveform. The combined waveform is then applied to the windings of the motor.
According to yet another broad aspect of the invention, a disk drive product of the type having a polyphase dc motor for rotating a data containing media is presented. The disk drive product has a circuit for generating a user-defined waveform first signal in synchronism with a reference frequency signal. A reference frequency signal generator generates pulses of a second signal at a reference frequency. A commutation circuit combines segments of the first and second signals, and a circuit applies the combined sinusoidal waveforms to coils of the polyphase dc motor in a commutative sequence.
According to still another broad aspect of the invention, a disk drive product of the type having a three-phase dc motor for rotating a data containing media is presented. The disk drive product has a plurality of circuits, each for driving a respective set of motor coils during a motor phase. Each of the plurality of circuits has a current waveform generator for generating a user-defined waveform first signal in synchronism with a reference frequency signal. A commutation state machine generates commutated motor driving second signals. A slew rate controlled power amplifier combines the first and second signals to produce driving signals to the respective set of motor coils.